1. Field of the Invention
This invention relates to improvements to an electrohotographic copying machine. Particularly, the invention relates to an improved hot-roll for fixing the toner images. Specifically, the invention relates to improvements to the surface material of the fixing roll which is useful in electrophotograpy.
2. Description of the Prior Art
There have been proposed several methods for flying reproduced images to paper in electrophotography. The most widely practiced method among them is the so-called "hot-roll method" using a heated roll to settle the images formed by a toner containing a fusible resin as the binder on a copying paper by heat-fusing. The hot-roll method has some advantages, for example, higher efficiency of fixing, use of relatively low temperatures with decreased possibility of problems arising avoidance of the need to use special chemicals, and compact construction of the copying machines. Hitherto, a metal roll whose surface is coated with a fluoropolymer or a room-temperature-vulcanizing silicone rubber has been used as the fixing roll for the hot-roll method because of their excellent thermal stability and releasability. See U.S. Pat. Nos. 3,268,351 (Moser) and 3,498,596 (Van Dorn).
However, the fixing rolls coated with a fluoropolymer are disadvantageous because it is necessary to repeatedly apply a release agent over the surface of the fluoropolymer layer during copying runs to augment or make up the insufficient release property inherent in the fluoropolymer itself. Accordingly, the copying machine provided with the fluoropolymer-coated hot-roll should have an additional device for the application of the release agent, which results in rendering the construction of the machine more complicated.
On the other hand, the hot-rolls coated with a room-temperature-vulcanizing silicone rubber can exhibit an excellent release property in the early part of the copying runs. Such release property then rapidly decreases and, as a result, the range of temperature for the fixing step becomes steadily narrower, thus the fixing rolls coated with a room-temperature-vulcanizing silicone rubber have proven to have a shorter copy-life and have to be subjected to frequent renewal or replacement in order to avoid the so-called off-set phenomenon, i.e., transfer of the toner to the surface of the fixing roll, or winding of the copying paper around the roll.
Under the circumstances, improvements in the surface material of the fixing roll have been considered to be an important factor to increase the efficiency of electrophotographic copying.